PROJECT SUMMARY Over 36 million people are living with human immunodeficiency virus (HIV) and it infects over 1 million new people every year. Several advancements in treatment and prevention have helped reduce HIV incidence and AIDS-related deaths, but are insufficient to stem the spread of the epidemic. An effective HIV vaccine is needed to help reduce new HIV infections. As demonstrated by the RV144 Thai Vaccine Trail, a successful HIV vaccine will need to induce both antibodies to prevent acquisition and a cellular immune response to control breakthrough virus. Rhesus cytomegalovirus, strain 68-1, expressing SIV antigens (RhCMV/SIV) enables stringent control of SIV replication in 50% of vaccinated rhesus macaques. This vaccine protects animals by inducing effector memory T cells that reside at the portals of entry. While the CD8+ T cell response in vaccinated rhesus macaques is well-characterized and highly unconventional, it does not directly correlate with protection. In contrast, little is known about the RhCMV/SIV-induced CD4+ T cell response. Because the rhesus macaque is outbred and diverse, their MHC II alleles are complex and prevent controlled studies of the CD4+ T cell response. In contrast, Mauritian cynomolgus macaques have limited genetic diversity and we can identify MHC-II-matched MCM. Therefore, we will vaccinate MHC-II-matched MCM with CyCMV expressing SHIV antigens to study the CD4+ T cell response. We predict that the frequency and function of the SHIV-specific CD4+ T cell response will correlate with post-infection SHIV control. In addition, CMV vaccine vectors for HIV do not protect against acquisition and do not elicit neutralizing antibodies. Therefore, we intend to induce antibodies in conjunction with CyCMV vaccination by combining CyCMV with sequential HIV-Env DNA/protein immunizations. We hypothesize that CMV vaccine regimens can be enhanced to protect against acquisition by the induction of HIV-Env specific antibodies and can protect against a novel SHIV challenge virus. In conclusion, we propose to enhance our understanding of CMV vaccine vectors and to increase the protective efficacy of this vaccine regimen. These findings will be directly applicable to impending clinical trials of CMV vaccine vectors in humans for HIV.